It is becoming increasingly common to forgo traditional, proprietary storage devices, both NAS and SAN, and instead using off the shelf hardware and installing a storage operating system on it for, what many call, “do it yourself” storage servers. This, of course, is a misnomer since no one calls a normal file server “do it yourself” just because you installed Windows yourself. Storage has a lot of myth and legend swirling around it and people often panic when the they think of installing Windows and calling it NAS rather than calling it a file server. So, if it makes you feel better, use terms like file server or storage server rather than NAS and SAN – problem solved. This is a part of the “open storage” movement – moving storage systems from proprietary to standard.
Choosing the right operating system for a storage server is important and not always that easy. I work extensively in this space and people often ask me what I recommend and the recommendations vary, based on scenario, and often seem confusing. But the factors are actually relatively easy, if you just know the limitations that create the choices and paths in the decision tree.
Before choosing an OS we must stop and consider what our needs are going to be. Some areas that need to be considered are: capacity, performance, ease of administration, budget, connection technology, cost and clustering. There are two main categories of systems that we will consider as well, standard operating system or storage appliance operating system. The standard operating systems are Windows, Linux, Solaris and FreeBSD. The storage appliance operating systems are FreeNAS, OpenFiler and NexentaStor. There are others in both categories but these are the main players currently.
The first decision to be made is whether or not you or your organization is comfortable supporting a normal operating system operating in a storage server role. If you are looking at NAS then simply ask yourself if you could administer a file server. Administrating a block storage server (SAN) is a little more complex or, at least, unusual, so this might induce a small amount of concern but is really in line with other administration tasks. If the answer is yes, that using normal operating system tools and interfaces is acceptable to you, then simply rule out the “appliance” category right away. The appliance approach adds complexity and slows development and support cycles, so unless necessary is undesirable.
Storage appliance operating systems exist only to provide a pre-packaged, “easy to use” view into running a storage server. In concept this is nice, but there are real problems with this method. The biggest problems come from the packaging process which pulls you a step away from the enterprise OS vendors themselves making your system more fragile, further behind in updates and features and less secure than the traditional OS counterparts. It also leaves you at the mercy of a very small company for OEM-level support when something goes wrong rather than with a large enterprise vendor with a massive user base and community. The appliancization process also strips features and options from the systems by necessity. In the end, you lose.
Appliances are nice because you get a convenient web interface from which “anyone” can administer your storage. At least in theory. But in reality there are two concerns. The first is that there is always a need to drop into the operating system itself and fix things every once in a while. Having the custom web interface of the appliance makes this dramatically harder than normal so at the time when you most need the appliance nature of the system is when you do not have it. The second is that making something as critical as storage available for “anyone” to work on is a terrifying thought. There are few pieces of your infrastructure where you want more experience, planning and care taken than in storage. Making the system harder to use is not always a bad thing.
If you are in need of the appliance system then primarily you are looking at FreeNAS and OpenFiler. NexentaStor offers a compelling product but it is not available in a free version and the cost can be onerous. The freely downloadable version appears to be free for the first 18TB of raw storage but the license states otherwise making this rarely the popular choice. (The cost of NexentaStor is high enough that purchasing a fully supported Solaris system would be less costly and provides full support from the original vendor rather than Nexenta which is essentially repackaging old versions of Solaris and ZFS. More modern code and updates are available less expensively from the original source.)
FreeNAS, outside of clustering, is the storage platform of choice in an appliancized package. It has the much touted ZFS filesystem which gives it flexibility and ease of use lacking in OpenFiler and other Linux-based alternatives. It also has a working iSCSI implementation so you can use FreeNAS safely as either a NAS or a SAN. Support for FreeNAS appears to be increasing with new developments being made regularly and features being retained. FreeNAS offers a large range of features and supported protocols. It is believed that clustering will be coming to FreeNAS in the future as well as this has recently been added to the underlying FreeBSD operating system. If so, FreeNAS will completely eliminate the need for OpenFiler in the marketplace. FreeNAS is completely free.
OpenFiler lacks a reliable iSCSI SAN implementation (unless you pay a fortune to have that part of the system replaced with a working component ) and is far more out of date than its competitors but does offer full block-level real-time replication allowing it to operate in a clustered mode for reliability . The issue here being that the handy web interface of the NAS appliance does not address this scenario and if you want to do this you will need to get your hands dirty on the command line, very dirty indeed. This is expert level stuff and anyone capable of even considering a project to make OpenFiler into a reliable cluster will be just as comfortable, and likely far more comfortable, building the entire cluster from scratch on their Linux distribution of choice. OpenFiler is built on the rather unpopular, and now completely discontinued, rPath Linux using the Conary packaging system both which are niche players, to say the least, in the Linux world. You’ll find little rPath support from other administrators and many packages and features that you may wish access to are unavailable. OpenFiler’s singular advantage of any significance is the availability of DRBD for clustering, which as stated above, in nonsensical. Support for OpenFiler appears to be waning with new features being non-existant and, in fact, key features like the AFP have been dropped rather than new features having been added. OpenFiler is free but key features, like reliable iSCSI, are not. Recent reports from OpenFiler users are that even non-iSCSI storage has become unstable in the latest release and losing data is a regular occurrence. OpenFiler remains very popular in the mindshare of this industry segment but should be avoided completely.
If you do not need to have your storage operating system appliancized then you are left with more and better choices, but a far more complex decision tree. Unlike the appliance OS market which is filled with potholes (NexentaStor has surprise costs, OpenFiler appears to support iSCSI but causes data loss, features get removed from new versions) all four operating systems mentioned here are extremely robust and feature rich. Three of them have OEM vendor support which can be a major deciding factor and all have great third party support options far broader than what is available for the appliance market.
The first decision is whether or not Windows only features, notably NTFS ACLs, are needed. It is common for new NAS users to be surprised when the SMB protocol does not provide all of the granular filesystem control that they are used to in Windows. This is because those controls are actually handled by the filesystem, not the network protocol, and Windows alone provides these via NTFS. So if that granular Windows file control is needed, Windows is your only option.
The other three entrants, Linux, Solaris and FreeBSD, all share basic capabilities with the notable exception of clustering. All have good software RAID, all have powerful and robust filesystems, all have powerful logical volume management and all provide a variety of NAS and SAN connection options. Many versions of Linux and FreeBSD are available completely freely. Solaris, while free for testing, is not available for free for production use.
The biggest differentiator between these three OS options is clustering. Linux has had DRBD for a long time now and this is a robust filesystem clustering technology. FreeBSD has recently (as of 9.0) added HAST to serve the same purpose. So, in theory, FreeBSD has the same clustering options as Linux but this is much newer and much less well known. Solaris lacks filesystem clustering in the base OS and requires commercial add-ons to handle this at this time.
Solaris and FreeBSD share the powerful and battle tested ZFS filesystem. ZFS is extremely powerful and flexible and has long been the key selling point of these platforms. Linux’ support for filesystems is more convoluted. Nearly any Linux distribution (we care principally about RHEL/CentOS, Oracle Unbreakable Linux, Suse/OpenSuse and Ubuntu here) supports EXT4 which is powerful and fast but lacks some of the really nice ZFS features. However, Linux is rapidly adopting BtrFS which is very competitive with ZFS but is nascent and currently only available in Suse and Oracle Linux distros. We expect to see it from the others soon for production use but at this time it is still experimental.
Outside of clustering, likely the choice of OS of these three will come down primarily to experience and comfort. Solaris is generally known for providing the best throughput and FreeBSD the worst. But all three are quite close. Once BtrFS is widely available and stable on Linux, Linux will likely become the de facto choice as it has been in the past.
Without external influence, my recommendation for storage platform are FreeBSD and then Linux with Solaris eliminated on the basis that rarely is anyone looking for commercial support and so it is ruled out automatically. This is based almost entirely on the availability of Copy-on-Write filesystems and assuming no clustering which is not common. If clustering is needed then Linux first then FreeBSD and Solaris is ruled out, again.
Linux and FreeBSD are rapidly approaching each other in functionality. As BtrFS matures on Linux and HAST matures on FreeBSD they seem to be meeting in the middle with the choice being little more than a toss up.
There is no single, simple answer. Choosing a storage OS is all about balancing myriad factors from performance, resources, features, support, stability, etc. There are a few factors that can be used to rule out many contenders and knowing these hard delimiters is key. Knowing exactly how you plan to use the system and what factors are important to you are important in weeding through the available options.
Even once you pick a platform there are many decisions to make. Some platforms include multiple file systems. There is SAN and NAS. There are multiple SAN and NAS protocols. There is network bonding (or teaming, the Windows world.) There is Multipathing. There are snapshots, volumes, RAID. The list goes on and on.